I have been redoing and redoing the Dutch lessons infinitely. I really need new lessons to make progress, but it seems that you cannot offer any more challenges, which is disappointing.
The Dutch team are currently working on Tree 2.0, which will include many new lessons, as well as improvements to old ones. You can follow updates on the new course from the Dutch Incubator page, or read the announcement post here. It is not currently known when the new tree will be released.
Since you've reached this stage of your learning, it could also be time to consider moving on from Duolingo. Duolingo and the Dutch course are fantastic, but they can only take you so far in your learning. If you are interested in other online courses, you could try Babbel, Memrise, learndutch.org or Taal Thuis. If you are interested in purchasing a textbook, I can personally recommend Complete Dutch, Dutch for Self-Study - Nederlands voor Zelfstudie and Dutch with Ease (I've only used the German version of this book, Niederländisch ohne Mühe, though they both have the same author, so I presume they are much the same). If you are looking for a language partner, try iTalki or Interpals for some practice with native speakers.
Building off what Alex said, another thing you could do (besides what was already mentioned), is pick up a Dutch book and start reading it. Then when you come across words you don't know, you can translate them using google translate or http://www.mijnwoordenboek.nl/ (mijnwoordenboek has much better translations from my experience). Once you've translated them, you could make your own Memrise course or Tinycards deck to review them.
Success met je Nederlands!